The invention relates generally to lightwave communication networks and, more particularly, to controlling crosstalk-related impairments in wavelength division multiplexed signals in such networks.
Recent advances in communication technologies are enabling service providers to meet the increasing demands being placed on today""s networks. For example, optical fiber is fast becoming a transmission medium of choice for many communication networks because of the speed and bandwidth advantages associated with optical transmission. In addition, wavelength division multiplexing (WDM) is being used to obtain even more bandwidth and speed in today""s optical transmission systems. In its simplest form, WDM is a technique whereby parallel data streams modulating light at different wavelengths are coupled simultaneously into the same optical fiber. As such, WDM offers the capability to combine many xe2x80x9coptical channelsxe2x80x9d each at a different wavelength for simultaneous transmission as a composite optical signal in a single optical fiber. By using optical transmission and WDM in the backbone networks, the communications industry has made great strides in terms of bandwidth and speed.
However, the features of WDM which give rise to the many advantages also present problems that must be addressed by system and network designers. For example, a common approach to increasing bandwidth is to increase the number of optical channels in the WDM signal and to decrease the spacing between channels. As a result, one problem that has to be addressed is adjacent channel crosstalk. In particular, crosstalk can be a significant problem in a WDM system because an optical channel can be affected by adjacent channels due to the high power being transmitted in each of the closely-spaced channels. As such, many components and systems are designed according to particular specifications with regard to crosstalk suppression. As an example, some optical demultiplexers, which are used to separate a WDM signal into its constituent wavelength channels, have a crosstalk suppression requirement meaning that the device is designed to suppress power in adjacent channels by a prescribed amount, e.g., 22 db, relative to the channel of interest.
To date, bit error rate (BER) performance has been the principle factor when determining appropriate crosstalk suppression design requirements for components. In particular, the bit error rate performance of an optical channel can be adversely affected by the crosstalk contribution from adjacent channels. As such, components such as optical demultiplexers are designed to ensure that crosstalk is suppressed by a prescribed amount to ensure acceptable bit error rate performance. Although acceptable bit error rate performance is a critical parameter for optical transmission, we have discovered that existing crosstalk suppression requirements do not effectively address other crosstalk-induced impairments in WDM systems.
By substantially reducing or eliminating crosstalk contribution from adjacent channels in a WDM optical signal according to the principles of the invention, the presence or absence of an optical channel can be accurately detected by an optical receiver. As a result, channel mis-identification, which might otherwise result from the detection of adjacent channel crosstalk signal power exceeding the optical receiver""s sensitivity threshold, is substantially avoided.
More specifically, we have discovered that crosstalk contribution from adjacent channels gives rise to a problem when a particular channel is out of service for any number of reasons, e.g., circuit pack failure or removal, automatic power shutdown mechanisms, inadvertent path or equipment disruptions, and so on. In particular, any of these above conditions typically cause a loss of signal (LOS) condition for the affected channel whereby power in that channel is either completely shut off or substantially reduced thus resulting in the receiver losing synchronization with the received signal. Many WDM systems have recovery mechanisms for restoring service whereby the loss of signal condition is cleared once the receiver locks back onto a received signal. Unfortunately, crosstalk from adjacent channels can cause a receiver in the failed channel to actually lock onto an adjacent channel because crosstalk power in the adjacent channel is above the receiver sensitivity threshold in the failed channel. Typically, a receiver will lock onto a signal as a function of the signal attaining a prescribed bit error rate (BER) level. Therefore, the receiver improperly identifies crosstalk power from the adjacent channel as an indication that the failed channel has been restored. As a result of this channel mis-identification, the loss of signal condition is improperly cleared (e.g., false LOS recovery) and the traffic from the adjacent channel is now present on the originally failed channel. We refer to this crosstalk-induced impairment as xe2x80x9ccross-trafficxe2x80x9d.